Optical fiber cables are used to transmit information including telephone signals, television signals, data signals, and Internet communication. To preserve the integrity of the signal transported by optical fiber cables, certain design factors warrant consideration.
First, forces may develop on the optical fibers due to contact with rough, hard, or uneven surfaces within the optical fiber cables. Such contact, for example, may result from thermal cable expansion or contraction, which can cause microbending and macrobending effects. This, in turn, can lead to signal attenuation or signal loss. Layers of protective coatings and claddings around the optical fibers can help to reduce the forces that cause these unwanted effects.
Second, the optical fibers are typically coupled to the surrounding buffer tube in some way. This coupling prevents the optical fibers from pulling back inside the buffer tube as a result of processing, installation, handling, or thermally induced dimensional changes. Not only can these effects hamper accessibility to the fibers during connection operations (e.g., splicing), but also insufficient coupling can lead to excess and/or unevenly distributed optical fiber length (e.g., optical fibers accumulating in a confined space). Such accumulation may cause bending or otherwise force contact between the optical fibers and other cable elements, which can likewise lead to microbending and macrobending.
Third, optical fiber cables are typically used with electronic devices. If water intruding into the cables can spread (e.g., flow) along the length of the cables to these electronic devices, severe damage to the electronic systems may result. It is also thought that the formation of ice within an optical fiber cable can impose onto the optical fibers localized microbending-inducing forces or macrobending-inducing forces. Fillers and water-blocking layers within the cables can impede the movement of water within the cables and thereby limit such damage.
The undesirable effects of signal loss, coupling failure, and water damage can be reduced through the use of protective layers and coupling elements. The addition of these layers, however, can lead to larger cables, which are not only more costly to produce and store but also heavier, stiffer, and thus more difficult to install.
Manufacturers have typically addressed these problems by employing water-blocking, thixotropic compositions (e.g., grease or grease-like gels). For example, filling the free space inside a buffer tube with water-blocking, petroleum-based filling grease helps to block the ingress of water. Further, the thixotropic filling grease mechanically (i.e., viscously) couples the optical fibers to the buffer tube.
That usefulness notwithstanding, such thixotropic filling greases are relatively heavy and messy, thereby hindering connection and splicing operations. Consequently, filling greases carry certain disadvantages.
Various designs for dry cables have been developed to eliminate filling greases while providing water-blocking and coupling functions. For example, in a totally dry cable, filling grease may be replaced by a water-swellable element (e.g., tape or yarn carrying a water-swellable material).
Unfortunately, in practice, the water-swellable elements used in these designs may not provide for sufficient coupling of the optical fibers to the buffer tube. That is, the optical fibers are free to pull back inside the cable when the cable is installed or exposed to temperature extremes.
Purported solutions to this problem have been proposed, typically involving the inclusion of a cushioning material, such as polymeric foam (e.g., polyurethane foam), that either surrounds the optical fibers or is layered on the water-swellable tape. To achieve the desired mechanical coupling, though, the cushioning is sized such that it is compressed between the optical fibers and the buffer tube. In this way, the cushioning promotes frictional coupling of the optical fibers to the buffer tube.
Although frictional coupling is effective in preventing relative movement between the optical fibers and the buffer tube, the optical fibers may experience microbending or macrobending when the buffer tube contracts due to cooling. This may result in optical signal attenuation or signal loss. Further, the coupling pressure exerted on the optical fibers by the foam cushioning may diminish over time due to the relaxation or degradation of the polymeric foam.
Accordingly, there is a need for a dry optical fiber cable in which optical fibers are coupled to a buffer tube in a way that does not exert undue stresses on the optical fibers and is reliable over the life span of the cable.